HYDRODYNAMICS AND ACOUSTICS

2018 ◊ Volume 1 (91) ◊ Issue 2 ◊ p. 117-131

A. A. Baskova^*, G. A. Voropayev^*

^* Institute of Hydromechanics of NAS of Ukraine, Kyiv, Ukraine

The structure of the vortex nonisothermal flow at the initial section of the pipe with transient Reynolds numbers

Gidrodin. akust. 2018, 1(2):117-131

https://doi.org/10.15407/jha2018.02.117

TEXT LANGUAGE: Russian

ABSTRACT

A direct numerical simulation of the flow structure of nonisothermal media on the initial sections of smooth pipes and pipes with corrugated inserts of various geometries is carried out. The emergence and development of wave and vortex perturbations in a laminar flow in a wide range of transient Reynolds numbers is studied. It is shown that vortex structure of the flow depends on the thermal nonequilibrium of the medium and on the geometry of the corrugated tube surface at transient Reynolds numbers. An intensification of the disturbances in a nonisothermal flow (when comparing with an isothermal flow with the similar Reynolds number) is observed due to negative medium viscosity gradient with respect to pipe surface. An analytical expression for the velocity profile is obtained. This last depends on the medium dynamic viscosity gradient which has the inflection point inside the thermal boundary layer and satisfies the necessary condition of flow instability with decreasing dynamic viscosity of the medium. The velocity profile in a dropping liquid flow loses its stability earlier in the presence of the cold pipe wall, while the gas flow, in the presence of the hot wall. The dependence of time and spatial scales of the vortex structure in the flow on Reynolds number and wall surface corrugation parameters are determined from the numerical experiment. For certain ratios of surface waviness dimensionless lengths and amplitudes referenced to pipe radius, the corrugated inserts are shown to be either the flow stabilizers in the initial pipe section, or the generators of low-frequency disturbances at the corresponding Reynolds number leading to the early transition to turbulence.

KEY WORDS

nonisothermal flow in a pipe, corrugation, gradient of dynamic viscosity, intensification of disturbances, vortex flow structure, initial stage of transition, hydraulic loss

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